1. Technical Field
The present invention relates to a latent image forming member, such as an ID card, and more particularly to a latent image forming member having a latent image pattern, such as a bar code having fluorescent grains dispersed and held therein, and a method of manufacturing the same.
The present invention also relates to a latent image reading apparatus for reading data recorded in the form of a latent image of a fluorescent material which emits light when it is stimulated by infrared rays, from an object on which the latent image has been formed. More particularly, the present invention relates to an arrangement and characteristics of an infrared ray source for irradiating a latent image with infrared rays and of a light detection device for detecting excited light from the latent image.
The present invention further relates to an information recording medium having a latent image mark and a system for reading the same, and more particularly to a configuration of the latent image mark and of an optical system of a reading apparatus.
2. Discussion of Related Art
In recent years, control of articles by means of bar codes has been performed widely in various industrial fields while centering the PDS (Point Of Sales) in the distribution field. Along with this, a suggestion has been made that a non-visible bar code, chat is, a latent image bar code is put on in terms of improving design and security (see, for example, Japanese Patent Application Laid-Open No. 52-80906).
As for manufacturing of the latent image bar code, a method has been suggested (see, for example, Japanese Patent Application Laid-Open No. 53-9607) in which a ribbon containing fluorescent grains is made to be transferred to paper upon impact so that the latent image bar code is printed on to the paper.
At present, if the latent image bar code of the foregoing type is formed on an ID card for use as a credit card or the like, there arise a problem of unsatisfactory durability, such as the wear resistance and chemical resistance against, for example, alcohol. It will come to be difficult to read the latent image bar code as the card is repeatedly used. Therefore, the code of the foregoing type has a disadvantage that the card cannot be used for a long time.
Further, a latent image reading apparatus has been known (see for example Japanese Patent Application Laid-Open No. 52-80906 and Japanese Patent Application Laid-Open No. 53-9600) in which data recorded in the form of a latent image of a florescent substance that emits light when it is stimulated by infrared rays, is read from paper on which the latent image has been formed.
The latent image can be obtained by printing, for example, on white paper or plastic, with ink which is obtained by mixing 200 g of sodium yttrium fluoride (NaYF.sub.4) fluorescent substance activated by, for example, ytterbium and erbium with 160 g of vinyl chloride type resin and 40 g of plasticizer, followed by dispersing the mixture in a 400 g of solvent. When the latent image of the foregoing type is irradiated by infrared rays, for example, a gallium arsenide (GaAs) infrared ray emitting diode, the ink emits light, the excited light having a wavelength different from that of the infrared ray. The reading apparatus for reading the latent image receives the excited light from the ink, and treat or process a signal of the light to read out the data recorded in the latent image.
Since the excited light obtainable from the latent image is very weak, if infrared rays from the infrared ray source is not efficiently applied to the latent image or if the excited light from the latent image is not efficiently incident upon a light detection device disposed in the reading apparatus, a high level of reading signal cannot be obtained and reading error is liable to occur.
In order to overcome a problem of the foregoing type, a conventional reading apparatus has been formed, as shown in FIG. 17 and FIG. 18, in such a manner that an excellent-directivity infrared ray source and a light detection device are used.
FIG. 17 is a view which illustrates an example of a conventional latent image reading apparatus mainly composed of a card 101 having a latent image 102 printed on the top surface thereof, an infrared ray source 103 for irradiating the latent-image printed portion of the card 101, a stem 104 for holding the infrared ray source 103, a light detection device 105, a filter 106 disposed in front of a light receiving surface of the light detection device 5 and permitting only light emitted from the latent image 102 to mass therethrough, a light-conductive member 107, such as an optical fiber, which leads light emitted from the latent image 102 to the filter 106, and a terminal 108 for reproducing data from an output signal from the light detection device 105. In this latent image reading apparatus, the stem 104 is disposed in a manner inclined relative to the normal direction of the card 101 in order to dispose the infrared ray source 103 adjacent to the card 101 as much as possible while preventing an interference with the conductive member 107. It is advantageous to minimize the inclination angle .theta. to improve the light emitting efficiency and the light receiving efficiency. However, a large inclination angle is actually likely to become larger than 30 degrees to 40 degrees. Further, the latent image reading apparatus is unable to efficiently lead the light emitted from the latent image 102 into the light detection device 105 by disposing the light-conductive member 107 extending from a portion where the card 101 is passed to a portion where the filer 106 is disposed.
FIG. 18 is a view which illustrates a second example of a conventionally known latent image reading apparatus. This apparatus has an arrangement to improve the efficiency of irradiating the latent image-printed portion of the card 101 by the infrared ray source 103 by coaxially disposing a collimator lens 109 and a focusing lens 110 in front of the infrared ray source 103. Since the residual portions are the same as those of the apparatus shown in FIG. 17, the corresponding portions are given the same reference numerals and their descriptions are omitted here.
An actual apparatus is assembled by a process in which a mold serving as a member holder is made by a plastic molded part, followed by inserting and fixing members such as the infrared ray source 103, the stem 104, the light detection device 105, the filter 106 and the light-conductive member 107. Therefore, the conventional apparatus has a vertical hole and an inclined hole so that the light detection device 105, the filter 106 and the light conductive member 107 are coaxially inserted into the vertical hole to be fixed therein, while the infrared ray source 103 and the stem 104 (and further the collimator lens 109 and the focusing lens 110 in the apparatus shown in FIG. 18) are coaxially inserted into the inclined hole to be fixed therein.
However, an operation for inserting the infrared ray source 103 and the stem 104 into the inclined hole suffers from an excessively unsatisfactory working efficiency as compared with an operation for inserting the other members into the vertical hole. Further, the fact that the infrared ray source 103 is fastened in the inclined manner necessitates that its lead wire should be formed to be bent vertically upwards at the time of connecting the infrared ray source 103 to a power supply or the like (omitted from illustration). From this view point, the assembling work is troublesome. Further, the number of parts is too large to easily complete the assembling work. As a result, the conventional latent image reading apparatus has disadvantages of unsatisfactory assembling and mass productivity, resulting in a problem of high cost. In addition, the conventional latent image reading apparatus comprises the conductive member 107 as a necessary element. Therefore, a length L from the light receiving surface of the light detection device 105 to the latent image forming surface of the card 101 becomes long, and therefore, thickness of the apparatus becomes large.
Also a system for reading a non-visible latent image mark formed by printing a bar code or the like with ink made of fluorescent material is disclosed in Japanese Patent Application Laid-Open No. 1-211089 or Japanese Patent Application Laid-Open No. 53-9600.
FIG. 50 is a schematic view which illustrates a usual latent mark reading apparatus. As shown in FIG. 50, a latent image mark 401 including bar-shaped fluorescent members is printed on a sheet 400. The latent image mark 401 is sequentially read when the sheet 400 is passed through a conveyance passage A3.
That is, a light source 402 for exciting the latent image marks 401 to emit light is disposed on a light irradiation path B3. In front of the light source 402, a filer 403 for cutting visible components radiated from the light source 402 is disposed.
Further, a light receiving device 404 is disposed on a light emitting path C3, and a band pass filter 405 is disposed in front of the light receiving device 404 to receive only light emitted from the latent image mark 401.
In FIG. 50 a slit plate 406 is disposed between the filer 403 and band pass filter 406 and the paper conveyance passage A.
As shown in FIG. 51, a multiplicity of bar-shaped latent image marks 401 are printed on the sheet 400. By combining the number of the marks 401 within a predetermined area and intervals between marks 401, desired information can be formed.
In a case where the sheet 400 having the latent image marks 401 is passed through the sheet conveyance passage A, the light receiving device 404 transmits an output having a waveform, as shown in FIG. 52A. However, an end of the slit adjacent to the light source blocks the excited light, and therefore the light emitted from the latent image marks 401 cannot be sufficiently utilized. Therefore, if a width of the latent image mark 401 is narrowed, the output from the light receiving device 404 is reduced. As a result, an S/N may become too low to realize a satisfactory operation reliability.
In order to improve the apparatus, it might be considered feasible to bring the slit 406 closer to the latent image marks 401 or to bring the light receiving device 404 closer to the same. If the slit 406 is brought closer to the latent image mark 401, at a portion where the latent image marks 401 are positioned close to each other, the output is undesirably enlarged, as is designated by portions X3 shown in FIG. 52B, before the basic output is restored. If a pulse conversion in a slice level is performed, pulse width P400 and pulse width P.sub.401 become different from each other, as shown in FIG. 52C. In this case, information cannot correctly be read. If the predetermined area includes a larger quantity of information by narrowing the intervals between the latent image marks 401, the foregoing tendency becomes stronger, resulting in a long pulse to be formed depending on circumstances to lead to erroneous detection.
This disadvantageous phenomenon similarly takes place in a case where the light receiving device 404 is brought closer to the latent image marks 401.
On the other hand, in a case where a reflecting type structure is employed, the output from the light receiving device 404 is, if the slit width is narrower than the mark interval, restored to the base level as shown in FIG. 52D, even if the slit 406 is brought close to the latent image marks 401. However, since the reflecting type structure transmits a reverse output waveform to that of the light emission type structure, the waveform is reversed.
The reason why the waveform in a case where the fluorescent member is used is different from that in the case of the reflecting type structure is as follows. As shown in FIG. 53, excited light in the form of plane-emitted light, irradiates an area wider than the slit and the fluorescent member emits light uniformly in all directions. Since the light receiving device 404 also has a somewhat wide light receiving area, the device 404 can receive light also from a position designated by a dashed line shown in FIG. 53. Further, light reflected by the slit plate 406 and the sheet 400 may happen to reach the light receiving device 404. These are the cause of the phenomenon shown in FIG. 52B.
It should be noted that Japanese Patent Application No. 3-163694 (U.S. patent application Ser. No. 07/722,186 and European Patent Application No. 9108120), claiming priorities of Japanese Patent Application No. 2-173655, Japanese Patent Application No. 2-178207 and Japanese Patent Application No. 2-282881 have disclosed technologies, about a detection mark, a method of detecting the mark and a detection apparatus, for the purpose of stably detecting the shape of the mark formed on a card while maintaining an excellent measuring accuracy.